Magnetic signal reproducing means



Feb. 25, 1947. 5, BEGU'N 2,416,279

MAGNETIC SIGNAL REPRODUCING MEANS Filed Jan. 22, 1943 AMPLIFIERFREQUENCY 'FlliB FIBB AMPLIFIIR HIGH PAH FILTER INVENTOR. .ffN/ JOJTPHBfGU/Y Patented Feb. 25, 1947 MAGNETIC SIGNAL REPRODUCING MEANS SemiJoseph Begun, Cleveland Heights, Ohio, as-

signor to The Brush Development Company, Cleveland, Ohio. a corporationof Ohio Application January 22, 1943, Serial No. 473,165

17 Claims. 1

My invention pertains to magnetic transducers and more particularly tomagnetic reproducing heads adapted to operate with a moving magnetizedmaterial and to electrical circuits connected to the reproducing heads.

An object of my invention is to extend the range of the frequencyresponse of magnetic systems.

Another object of my invention is to provide magnetic signal reproducingmeans having an improved low frequency response.

A further object of my invention is to provide a circuit for use with acombination or double reproducing head.

It is also an object of my invention to provide a circuit for use with acombination reproducing head which provides a relatively flat overallsignal response.

Another object of my invention is to provide a combination reproducinghead wherein the phase relation and the signal level of the signalresponse of the individual heads are designed to prevent dips and peaksin the overall signal response from the combination head.

Other objects and a fuller understanding of my invention may be had byreferring to the following description taken in conjunction with thedrawings in which:

Figure 1 shows a combination reproducing head.

Figure 2 is a plan .view of one of thetwo heads of the combination head.

Figure 3 is a sectional view taken along line 3-3 of Figure 2, andlooking in the direction of the arrows.

Figure 4 is a graphical representation of the frequency response of eachof the two heads which form the combination head.

Figure 5 is a graphical representation of the overall response of thecombination head.

Figure 6 shows my improved combination reproducing head.

Figure 7 is a graphical representation of the response of my improvedcombination reproducing head.

Figure 8 shows another form of my invention.

It has been found in magnetically recording and reproducing signals witha magnetic head (i. e. a pole piece surrounded by a coil)- that afrequency range of about 1 to 100 can be easily handled. That is, if thelowest frequency is about 100 cycles per second the highest frequencywould be about 10,000 cycles per second. This ratio holds to a more orless marked degree over a fairly wide range of speeds of themagnetizable material. A magnetic reproducing head "reproduces" arecorded signal by establishing a voltage in accordance with therecorded magnetic signal. The voltage established is a function of therate of change of flux through the magnetic head. For a given tape speedthe flux through the reproducing head is a function of the frequencywhich was recorded on the tape. Below a certain frequency the rate ofchange of flux in the magnetic head drops sharply, thereby resulting ina drop in the generated voltage. This is the low frequency limitation ofa magnetic reproducing head. The high frequency limitation arises whenthe length of the wave being reproduced approaches the finite thicknessof the pole piece, thus generating voltages which tend to cancel eachother and drop the output of the head.

It is desirable in very many cases to extend the frequency range inreproduction beyond the above mentioned ratio of 1 to 100. This I do byproviding two reproducing heads in association with the movingmagnetized material, and by specially designing one of the heads topermit it to reproduce frequencies which are lower than the lowest whichthe other head can reproduce. Each of the two heads reproduces thesignal which is on the magnetized material, and the outputs from thetwoheads are mixed to establish a single combination or overall outputhaving a frequency range which is extended at the lower end compared toa standard reproducing head.

Figure 1 shows the two reproducing heads l0 and II electricallyconnected in series and positioned close together and in associationwith a magnetized tape or wire l2. The outputs from the two heads areconnected to an amplifier [3. The reproducing head I!) is adaptedprimarily for reproducing low frequency signals, such for example asthose covered by the range indicated by the response curve ll of Figure4, and the reproducing head II is adapted primarily'for re-v producinghigher frequency signals, such for example as those covered by the rangeindicated by the response curve I5 of Figure 4.

As shown in Figure 1 the reproducing head It is a head comprised of twopole pieces l8, l1, one of which is surrounded by a coil l8. The coil itwhich surrounds the pole piece 16 is connected in series with thereproducing head l0 and the amplifier IS.

The low frequency reproducing head is shown in detail in Figures 2 andv3, and'comprises a. spool 25 having a shank portion 24, and a flange 26at one end and a flange 21 at the other end.

of turns of light 'wire 30 is wound on the spool.

The shaft 24 of the spool and the plug 29 prevent the moving tape I2from cutting into the wire 30. For best results the wire 30 should beJust as close to the magnetizable tape l2 as possible. For this reasonthe slot 28 through the spool should be just slightly wider than thetape l2, and the thickness of the spool shank 24 should be as small aspossible. Because the spool extends for some distance in the directionof movement of the tape l2 it is relatively non-responsive 1 f tosignals of short wave length, and is relatively more responsive tosignals of longer wave length than reproducing heads which utilize polepieces. Its excellence in picking up low frequency signals arises fromthe fact that it is not focused on a small portion of the tape like apole piece is, and the reason it is relatively non-responsive to thehigher frequency signals is that when the wave length of the signal tobe reproduced approaches the length of the head l there will becancelling voltages induced in the head, and consequently the outputwill be small. Thus, by combining two reproducing heads, one of whichpermalloy pole piece reproducing head ii, there is a rounded peak atabout 1500 cycles per second and a smooth fall of! to around 4000 cyclesper second. At the lower end ther is a sharp drop in response fromaround 100 cycles per second to 30 cycles per second. Theresponse curve.of the low frequency head l0 compared to the response curve of the headH would show that at about 30 cycles per second the response from thehead l0 would be about 15 d. b. higher than the response from the headII, and that the curve falls rather sharply to 200 cycles per second atwhich point the response is quite small. At around 80 cycles per secondthe responses from the twoheads are about the same.

The overall response curve 3'! taken, using the combined output fromboth heads "I and II,

' which were placed .46 of an inch apart along the is particularlyresponsive to low frequencies and the other of which is particularlyresponsive to higher frequencies, a composite or combination reproducinghead is formed which has a frewill, however, always be a certain finitedistance between the two heads, and it will occur for a certain speed ofthe magnetized material and for 3 a certain recorded frequency, that thevoltage generated in one pickup device is 180 degrees out of 3 phasewith the voltage generated in the other 3 pickup device. In such a casethe voltages buck each other, and if they are of the same magnil tudethey will cancel out leaving no resultant 1 output. For otherfrequencies, if the speed of a the magnetized material is assumed to beconstant, the voltages will be in phase and will add 1 to each othergiving an increase in the resultant .1 output. For frequencies betweenthose which i establish bucking voltages and those whichestablish'aiding voltages there will be a wide variation in the relativephase shift and therefore the resultant voltages will change dependingupon the comparative amplitude of the two reproduced signals. Thiscancelling and adding of voltages 3 is shown in the graphicalrepresentation in Figure 5, where the curve I4 represents the responsefrom the head I0 and the curve I5 represents the 3 response from thehead H. 1 for a certain frequency range the responses over- 5 lap. It isin this range that the voltages will 8 add and subtract to produce anundesirable overj all response curve such as is shown by curve 31.

It will be seen that An example of the responses may be as follows:

tape i2, shows a sharp minimum at about 94 cycles per second due to thespacial separation and interference between the two reproducing heads.There is also aslight reinforcement at 4'7 cycles per second and anotherat 141 cycles per second. There is also a cancellation at 188 cycles.Further reinforcements and cancellations do not show because the levelof one or the other of the heads is down too low.

Several things may be done to prevent or lessen the dips and peaks inthe overall response curve. One way of greatly improving the overallresponse is to cause the voltage generated by the'head ID to fall downfor frequencies which overlap with the frequency range of the head i l,and to cause the voltage generated by the head II to fall down forfrequencies which overlap with the frequency range of the head I0.Thiswould mean that the upper end of the response curve of the head [0would drop sharply, and that the lower end of the response curve of thehead ll would drop sharply. Such a drop in the frequency responses maybe accelerated by suitable networks, such for example as,resistance-condenser networks.

Figure 6 shows the use of these net-works. The output from the head [0passes through the low pass filter 43, then to the amplifier 44 wherethe level of the signal can be adjusted, and then to the mixer 45. Theoutput from the head I l passes through the highpass filter 46, then tothe amplifier 41 where the level of the signal canbe adjusted, and thento the mixer 45.

The output from the mixer 45 is across the terminals 48, which may beconnected to a loud speaker, transient analyzer, oscilloscope,- or anyother such device for utilizing the signal. By using a low pass filter43, the response curve of the head l0 will appear somewhat likecurve 52of Figure 7, and by using a high pass filter 46 the response curve ofthe head II will appear somewhat like curve 53. These two responsecurves fall sharply where they overlap. An overall response curve willappear somewhat like line 54, which has only several very small peaksand dips, and those extend over a very small frequency range. Thereasons why filters improve the overall response is that they narrow therange of interference and they also may shift the phase of the signals.The position of the filters and the amplifiers is immaterial as theamplifiers may just as well be placed before the filters as after. It ispossible to develop the net-works to shift the phase in a directionwhich will in effect reduce the spacial distance between the two heads.

Figure 8 illustrates a circuit for shifting the phase of the signal fromhead III to in effect reduce the spacial distance between head HI andhead II. The circuit includes a network consisting of resistor 55 andcondenser 56, and the resistor and condenser are so proportioned thatthe resistance of 55 is large compared to the reactance of condenser 56throughout the frequency range in which the responses of the two headsoverlap. With the elements so proportioned the signal delivered -toamplifier M is delayed an amount of time corresponding to one-quarter ofa wave-length, thus, in eifect reducing the spacial distance between theeffective center of coil l stood that the present disclosure has beenmade only by way of example and that numerous changes in the details ofconstruction and the combination and arrangement of parts may beresorted to without departing from the spirit and the scope of theinvention as hereinafter claimed.

I claim as my invention: v

1. A magnetic reproducing device for use with a magnetizable mediumcomprising, in combina-. tion, a first magnetic reproducing headassociated with said magnetizable medium for reproducing the longwave-length signals therefrom, a second magnetic reproducing headpositioned closely adjacent to said first magnetic reproducing head andassociated with said magnetizable medium for reproducing shorterwavelength signals, the length of the spacing between the said first andsecond reproducing heads corresponding to the length of one of the,shorter waves which is reproduced by said second head, amplifier means,and circuit means connecting said first and second reproducing heads tosaid amplifier means and including signal time delay means in the outputcircuit of the reproducing head which first reproduces a given recordedmagnetic signal.

2. A magnetic reproducing device for use with a magnetizable mediumcomprising, in combination, a first magnetic reproducing head associatedwith said magnetizable medium for reproducing the long wave-lengthsignals therefrom, a second magnetizable reproducing head associatedwith said magnetic medium for reproducing the short wave-length signalstherefrom, amplifier means, circuit means connecting the output fromsaid first reproducing head and the output from said second reproducinghead to said amplifier means, said circuit means including low passfilter means in the output circuit of said first reproducing head andhigh pass filter means in the output circuit of said second reproducinghead.

3. A device as set forth in claim 2 in which the upper limit of said lowpass filter and the lower limit of said high pass filter are set atsubstantially the same wave-length in the range covered by saidreproducing heads.

4. A magnetic reproducing device for use with a magnetizable mediumcomprising, in combination, a first magnetic reproducing head associatedwith said magnetizable medium for reproducing the long wave-lengthsignals therefrom, a second magnetizable reproducing head associ- 6 atedwith said magnetic medium for reproducing the short wave-length signalstherefrom, signal mixer means connected to the output signal circuitfrom each of said reproducing heads, and phase shifting'means positionedin the said output signal circuit from one of said reproducing means tosaid mixer means. I

5. A device as set forth in claim 4 in which said phase shifting meansis in the output circuit between the head which reproduces the lowwave-length signals and the mixer means.

6. A device as set forth in claim 4 in which said phase shifting meansis a signal integrating circuit.

'7. A magnetic reproducing device for use with a relatively movingmagnetizable medium comprising, in combination, reproducing head meansincluding a first magnetic reproducing'head associated with saidmagnetizable medium for reproducing the long wave-length signalstherefrom and surrounding a portion of the said magnetizable medium anda second magnetic reproducing head associated with said magnetizablemedium and spaced from said first head for reproducing shorterwave-length, signals, the length of the spacing between the said firstand second reproducing heads corresponding to the length of one of thesaid shorter length waves, amplifier means, and circuit means connectingsaid first and second reproducing heads to said amplifier means, saidcircuit means including means for delaying the signal reproduced by theone of said reproducing heads which first reproduces a given signal withrespect to the signal reproduced by the other reproducing head.

8. A device as setforth in claim '7, further characterized in that saidcircuit connected to said reproducing head means includes signal timedelay means which integrate the signal.

9. A device as set forth in claim 1, further characterized in that saidsignal time delay ,means which is in the output circuit of thereproducing head which first reproduces a given recorded magnetic signalis an integrating cirtion, a first magnetic reproducing head associatedwith said magnetizable medium for reproducing the long wave-lengthsignals therefrom, a second magnetic reproducing head associated withsaid magnetizable medium for reproducing the short wave-length signalstherefrom, signal mixer means connected to the output signal circuitfrom each of said reproducing heads, and a phase shifting integratingcircuit in the output circuit between the head which reproduces the longwave-length signals and the mixer means.

11. A magnetic reproducing device for use with a magnetizable mediumcomprising, in combination, a first magnetic reproducing head associatedwith said magnetizable medium for reproducing the long wave-lengthsignals from a given portion thereof, a second magnetic reproducing headassociated with said magnetizable mediumfor reproducing the shortwave-length signals from said given portion after said first magneticreproducing head has reproduced the long wavelength signals, signalmixer means connected to the output signal circuit from each of saidreproducing heads, and signal time delay means positioned in the outputsignal circuit from the said first magnetic reproducing head fordelaying its reproduced signal with respect to the v said reproducinghead.

signal reproduced by the said second magnetic reproducing head.

12. A magnetic reproducing device for use with a magnetizable mediumcomprising, in combination, a first magnetic reproducing head associatedwith said magnetizable medium for. repro ducing the long wave-lengthsignals therefrom,

a second magnetic reproducing head independent from said firstreproducing headand associated with said magnetizable medium forreproducing the short wave-length signals therefrom, signal-mixer meansfor combining at least two signals, circuit means connecting each ofsaid reproducing heads' to said signal .mixer means, and means in saidcircuit for modifying the signal reproduced by at least one of saidreproducing heads whereby the combined signal from said mixer means isaltered to improve its overall characteristics.

13. A ma netic reproducing device for use with a magnetizable mediummoving at a given rate of speed comprising, in combination, firstmagnetic reproducing head means including an output circuit in fluxlinkage relationship with said magnetizablemedium for reproducing thelong said first reproducing head means has a low pass filter in itsoutput circuit and said second reproducing head means has a high passfilter in its output circuit, the high frequency cut-off frequency ofsaid low pass filter and the low frequency cut-off frequency of saidhigh pass filter being substantially the same, and means for combiningthe output filters.

17. In a signal reproducing circuit for a magnetic reproducing devicehaving a magnewave-length signals therefrom, second magnetic which firstreproduces a given recorded signal,

the output of said phase shifting means being connected to the outputcircuit of said other of said magnetic reproducing head means.

14. A magnetic reproducing device as set forth in claim 13 furthercharacterized in this: that at least one of said reproducing head meanshas in its output circuit filter means for sharply cutting the output atone end of the frequency range of 15. A magnetic reproducing device asset forth in claim 13 further characterized in this: that said firstreproducing head means has a low pass filter in its output circuit andsaid second reproducing head means has a high pass filter in its outputcircuit, the high frequency cut-off frequency of said low pass filterand the low frequency cut-off frequency of said high pass filter beingsubstantially the same.

tizable medium moving at a given rate of'speed, first magneticreproducing, head means in flux linkage relationship with saidmagnetizable-n'u'adium for reproducing. signals therefrom which arewithin a' given frequency range andincludingan output circuit having twowires, second magnetic reproducing head means in flux linkagerelationship with said magnetizable medium' for reproducing signalstherefrom which are within a second given frequency range and includingan output circuit having two wires, a portion only of said secondfrequency. range overlapping a portion only of said first givenfrequency range whereby throughout the overlapping frequency range bothsaid head means reproduce signals, said first and second reproducinghead means being spaced apart from each other along said magnetizablematerial in the direction of motion of said material, and phase shiftingcircuit means in the output circuit of the one of said two reproducinghead means which first reproduces a given signal recorded on saidmagnetizable medium, said phase shifting circuit means comprising aresistor connected to one of the wires from the magnetic reproducinghead, and a condenser connected across said two wires from the magneticreproducing head, the resistance of said resistor being large comparedto the reactance of said condenser throughout the frequency rangein-which both of said heads reproduce signals.

SEMI JOSEPH BEGUN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,251,300 Star Aug. 5, 19412,317,199 Kirschbaum Apr. 20, 1943 I FOREIGN PATENTS Number Country Date656,831 German Sept. 13, 1932 8 in claim. 13 further characterized inthis: that signal from said two

